Nuclear Power Plant (NPP) is the power generating station using the energy generated from Nuclear Fission or Nuclear Fusion reaction to produce electricity.
In order to protect the health of working personnel and the around residents from being irradiated, the design, construction and operation of NPP all follow the defense-in-depth principle, thus multiple protection are provided through the devices and means to ensure the effective control of the output power of the reactors. When various natural disasters, such as earthquake, tsunami, flood, or artificially accidents caused by fire, explosion happen, the fuel assembly of the reactor could be cooled down adequately ensuring the radioactive material not emitting to the environment.
The power supply should reflect the defense-in-depth idea either from the configuration or operation since it acts as the source of power. In order to achieve the high reliability of the power supply of NPP, emergency power supplies should be equipped with the especially important power consumption devices or devices of special requirements, multiple and independent configuration should be done to avoid the failure of the emergency power supply in case of the common mode fault.
The emergency power supply system and the normal power supply system together constitute the station power system, providing safe and reliable power supply for all the power consumption devices.
Plenty of redundant power supplies are equipped with NPP, including dedicated emergency powers, e.g., out-station main power, out-station auxiliary power and emergency stationary diesel, which do their own duties and cooperate mutually. These emergency power supplies are not only in various forms but configured in layers and multiple redundant, providing the reliable power supply for NPP to the maximum degree.
Currently, the operation modes of the station power system are as followed:
Under the normal working conditions, the power distribution system of the whole station power system are power supplied by the 26 KV busbar of generating set through the high voltage station transformer;
When the generating set is in operation, 26 KV busbar is power supplied by the main generator;
When the generator is shutdown, 26 KV busbar is down power supplied by the 400/500 KV power grid through the main transformer;
In case that 26 KV busbar loses power supply or high voltage station transformer, i.e. out-station main power, 220 KV power grid provides power supply for the safety auxiliary devices which should remain operational through auxiliary transformer to maintain the reactor in thermal shutdown state;
In case that the out-station main power and out-station auxiliary lose power supplies, the stationary diesel generator should provide power supply to the emergency ancillary equipment which enables the reactor enter cold-shutdown state;
In case that any one set of the emergency diesel generators is disabled, the additional emergency diesel unit take the place to perform the function of the emergency diesel generators, to provide the power supply for the dedicated safety devices, removal of the resident heat of the reactor core and spent fuel pool.
However, there are some limitations to the stationary diesel generator units. In case of the power supply failure, as the final emergency power supply, the stationary diesel generator units could not survive the flooding disaster such as floods, tsunami, typhoon, etc., due to its characteristics. In case that the extreme natural disaster exceeding the design standard happens, the stationary diesel units are easy to lose the power supply capability, are incapable of providing the power supply for the removal of the resident heat of the reactor core and spent fuel pool, resulting in disastrous consequences.